Recently, due to environmental and health concerns over lead containing solder alloys, electronic and semiconductor industries have been rapidly converting to lead free solders. Among lead free solder alloys, tin-silver-copper (Sn—Ag—Cu) alloys have been considered the most promising. However, tin-silver-copper (Sn—Ag—Cu) alloys typically have a high liquidus temperature (i.e., equal or greater than 217° C., which is approximately 34° C. greater than the eutectic temperature of tin-lead solder alloys (e.g., Sn63Pb37)). Such a high liquidus temperature may result in thermal damage to electronic components and printed wiring boards (PWB), thereby resulting in yield loss and reduced reliability. Therefore, alloys (e.g., Sn—Zn, Sn—Ag—In, and Sn—Ag—Cu—In alloys) with lower liquidus temperatures (e.g., about 193-213° C.) have been considered more suitable than tin-silver-copper (Sn—Ag—Cu) alloys for applications sensitive to damage due to thermal excursions.
Unfortunately, tin-zinc (Sn—Zn), tin-silver-indium (Sn—Ag—In), and tin-silver-copper-indium (Sn—Ag—Cu—In) alloys tend to exhibit too great a yield strength and brittleness and thus have been deemed unsuitable for next generation electronic and semiconductor devices. Such next generation electronic and semiconductor devices employ brittle and porous low dielectric materials in silicon chips and thus require the ability to withstand significant impact and shock due to increasing popularity of portable electronic devices such as the cellular phones, personal data assistants (PDA), laptop computers, etc. In view of the above-cited issues, there has been a strong interest in developing low temperature compliant lead free solders for these demanding applications.
In particular, there has been an interest in developing a low liquidus temperature alloy having a compliance that is comparable to that of lead containing solder alloys such as, for example, Pb95Sn5, which was traditionally the most widely used compliant solder in the semiconductor and electronic industries. To that effect, alloy compositions, as well as methods of application, of tin-indium (Sn—In) alloys have been explored. These explorations revealed that solder compositions of 85-96% tin and 4-15% indium underwent a Martensitic transformation to provide ductile interconnects for flip chip applications. Other explorations further revealed that doping could refine solder grain size to retain a fine grain structure and result in superplasticity after significant thermal cycling in a semiconductor package. However, even with doping to achieve a fine grained tin-indium (Sn—In) solder and thus superplasticity, these tin-indium (Sn—In) solders still exhibited too low a compliance and too high a yield strength (i.e., 3400-3800 psi, or approximately 36-150% greater than that of Pb95Sn5, which has a yield strength of 2500 psi). In order to withstand high stresses generated by large mismatches of thermal expansion between a silicon chip and an organic substrate, and to withstand impacts occurred in portable devices due to dropping and mishandling, low yield strength is probably preferred. For example, an alloy with higher yield strength could transmit stresses to a silicon chip and cause fractures in the silicon chip, instead of relieving stresses by plastic deformation.
In view of the foregoing, it would be desirable to provide a technique for providing low temperature lead free alloys which overcomes the above-described inadequacies and shortcomings.